“The ordinance is specific to the Central Business District. It is a violation of city ordinance for anyone to remove any contents of any bin, bag or other container that has been placed for collection of garbage, trash or recyclable materials. (See the ordinance: Art. 1 Sec. 39-2, Houston Code of Ordinances). An officer has probable cause to issue such a citation when a person is seen opening a lid and rummaging through contents of a dumpster or trash can.”

But seems to me, with the City of Houston ordinance using the wording: “for anyone”, it also just made it illegal for police, FBI, etc. to search for evidence in trash cans too.

http://www.houstontx.gov/solidwaste/chapter39-2010may.pdf
Just read the actual ordinance (above). So Houston police have a "defense" as agents or employees of the city but the wording seems to still find the FBI not exempt from the misdemeanor.